1. Field of the Invention
The present invention relates to a method and an apparatus for aligning first and second objects with each other, and more particularly, to a method and an apparatus for aligning a mask and a wafer with each other during a projection/exposure process in the manufacture of semiconductor devices.
2. Description of the Related Art
In a projection/exposure process in the manufacture of a semiconductor device, an exposure light beam emitted from light source 1 is applied to a circuit pattern previously formed on mask 2, as shown in FIG. 1. An image of the circuit pattern is projected on wafer 4 after being reduced in size by means of projection lens 3. Thereupon, a resist of wafer 4 is exposed, so that the pattern image is transferred to wafer 4.
In order to transfer the image of the circuit pattern accurately to a predetermined portion of the wafer, the mask and wafer must be aligned with each other before the exposure light beam is applied to the mask. The TTL (through the lens) method is a major aligning method for this purpose. This method is characterized in that an alignment light beam, which has a wavelength different from that of the exposure light beam, is transmitted through projection lens 3. A method using two diffraction gratings is stated in some documents (by G. Dubroeucq, 1980, ME; W. R. Trutna Jr., 1984, SPIE), as an example of the TTL method. As shown in FIG. 2, diffraction gratings 5 and 6 are formed on mask 2 and wafer 4, respectively. An alignment light beam emitted from alignment light source (laser light source) 7 is diffracted along a path from diffraction grating 6 of the wafer to diffraction grating 5 of the mask. The intensity of the diffracted light beam is detected by means of detector 8. Since the diffracted light beam carries information on dislocation between the mask and wafer, the position of the wafer relative to the mask is detected as the intensity of the diffracted light beam changes.
It is to be desired that the wire of the circuit pattern should be as thin as possible, that is, resolution R=.varies. .lambda./NA should be minimized (.lambda.: wavelength of the exposure light; NA=sin.alpha., where .alpha. is half the angle at which the exposure light beam is converged on the wafer). Resolution R can be lessened by widening angle .alpha. or reducing .lambda.. Due to structural restrictions on the projection lens, however, half-angle .alpha. cannot be unlimitedly increased. It is advisable, therefore, to reduce wavelength .lambda. of the exposure light beam. Presently, a g-line light beam (436 nm) is utilized as the exposure light beam. For a thinner circuit pattern wire, however, an i-line light beam (365 nm) or Krf excimer laser beam (248 nm) is expected to be used as the exposure light beam in the future.
The resist of wafer 4 is sensitive to a light beam with a wavelength of 500 nm or less. Accordingly, a light beam with a wavelength exceeding 500 nm is used as the alignment light beam, in order to avoid affecting the resist. Currently, an He-Ne laser beam of 633-nm wavelength is the most prevalent light beam for the purpose. Even at present, therefore, the exposure light beam and the alignment light beam have different wavelengths. The difference between the two wavelengths, however, is expected to be increased in the future.
Meanwhile, the image of the circuit pattern should be formed focused on the wafer for accurate exposure thereon. Thus, the distance between the mask and wafer is set so that the exposure light beam from the mask can be converged by the projection lens to be focused on the wafer. In other words, the aberration of the projection lens is adjusted so as to be minimized only for the exposure light beam, that is, the projection lens has chromatic aberration for light beams of any other wavelengths than that of the exposure light beam.
In aligning the mask and wafer with each other, therefore, the diffracted alignment light beam from the mask cannot be focused on the wafer, and instead, is focused on a point at distance d from the wafer, as shown in FIG. 2. If a g-line beam (436 nm) is used as the exposure light beam, the distance between the mask and wafer ranges from about 600 mm to 800 mm, while distance d is only scores of millimeters.
Conventionally, ordinary engineers believes that the sensitivity of diffracted light beams to be detected is too low for a mask and a wafer to be aligned accurately with each other, unless the diffracted alignment light beam is focused on a mask mark. Therefore, prior art aligning apparatuses are provided with means for correcting the length of the optical length of the diffracted alignment light beam, as shown in FIG. 2. More specifically, return mirrors 9 are disposed in the middle of the path of the diffracted alignment light beam. The optical path of the diffracted alignment light beam is extended by the distance for which the diffracted alignment light beam passes between mirrors 9, so that the diffracted alignment light beam from the mask can be focused on the wafer. If the aligning apparatus is provided with such correction means, however, the apparatus will inevitably be complicated in construction.
If a Krf excimer laser beam, whose wavelength is extremely short (248 nm), is used as the exposure light beam, moreover, the difference between the wavelengths of the exposure light beam and the alignment light beam is very large. Therefore, the diffracted alignment light beam is focused on a point at distance D (several thousands of millimeters) from the wafer, as shown in FIG. 2. In this case, the return mirrors must be positively increased in size or complicated in construction, in order to correct the length of the optical path of the alignment light beam. Practically, therefore, it is impossible to correct to the optical path length by means of the return mirrors. Thus, if the wavelength of the exposure light beam is very short (i.e., if there is a great difference between the wavelengths of the exposure light beam and the alignment light beam), the mask and wafer conventionally cannot be aligned with each other.